This invention relates to devices for undergoing reciprocating linear motion at controlled speed, and particularly to dot matrix printers having reciprocating hammer banks.
It has been known for some time to reciprocate a printing head along a line of characters to be printed, particularly with single printing heads using long flexible wires that are arranged to print all dots in a vertical column concurrently. Lead screws, belt drives, and other mechanisms have been suitable for these purposes, because the primary movable element has usually been a relatively small guide structure, near the paper, for the wires. More recently, line printers of the dot matrix type have been introduced that use a bank of print hammers reciprocated back and forth along a printing line, with each hammer serially printing the dots in the horizontal rows of one or more characters during movement in one or both directions. Early versions of this printer employed a stepping drive which incremented the hammers between successive horizontal positions so as to insure the desired dot registration. The hammers were movable relative to a stationary bank of magnetic actuators which received the printing impulses.
It has been demonstrated, however, in U.S. Pat. No. 3,951,051, that substantial improvements in cost, system operation and versatility can be achieved by "on the fly" printing using reciprocation of a hammer bank structure incorporating the magnetic actuators. As described in U.S. Pat. No. 3,951,051, the hammer bank movement spans a number of character positions, and during movement across the character positions the hammer bank is driven at substantially constant speed. A position encoder responsive to hammer bank position is used to generate timing signals to demarcate the column locations within the dot matrix patterns. Because the characters are small and the dot matrix patterns must be spatially reproduced with accuracy for good character readability, predictable speed control and precise position indexing are required for good print quality. The speed across the row of printing positions need not be uniform, but is preferably so in order to achieve maximum data throughput rates. Thus in the referenced system, a trapezoidal velocity function is adhered to, with the hammer bank being held at constant speed in one direction, then uniformly decelerated to a stop and accelerated to a constant speed in the opposite direction.
The acceleration and deceleration rates are high, as is the speed in each direction considering the short travels involved. With a printer operating at 40 to 400 lines per minute and each hammer printing 3 to 6 characters in a 5.times.7 dot pattern, the hammer bank must be reciprocated of the order of 10-50 times per second at 10-30 inches per second over a distance of 0.3 to 0.6 inches.
In the system of the referenced patent, the desired hammer bank shuttle motion is imparted by a counterbalanced, cam controlled positive drive mechanism. The mechanism has sufficient mass and drive power to maintain substantially constant speed despite the variable braking effect that is introduced during printing and the effect of spring loaded cam follower bearings. The controlling cam surface must be precisely generated for the desired trapezoidal motion, although substantial wear can have an adverse effect on the nature of the motion. With this arrangement, a large drive motor and flywheel are desirable for stability, and there are practical limitations on the shuttle rate that can be achieved. These factors together establish certain cost/performance criteria which represent a significant part of the cost of the overall system. Thus for particular applications, including both lower and higher line printing rates, it would be highly advantageous to employ a drive mechanism that would afford substantial economies in cost if the needed performance could be achieved.